1. Field of the Invention
The present invention relates to a method and apparatus for measuring the dry time of ink, and, more particularly, to a method and apparatus for measuring the dry time of ink in an ink jet printer.
2. Description of the Related Art
Ink jet printers are known to have a print quality problem related to ink dry time. Suppose that a page is printed and ejected from an ink jet printer. A second page is then printed and ejected onto the first page. If the ink from the first page is not yet dry, then the ink on the first page will smear, and/or some portion of the ink will transfer to the back of the second page.
It is known to provide mechanical devices to keep the second page from touching the first page for as long as possible. After the second page is printed, the mechanical device allows the second page to fall gently onto the first page. A problem is that smearing can still occur if the ink on the first page is not yet dry.
It is also known to blow air across the first page, in an attempt to accelerate evaporative drying. This method adds cost and size to the printer.
Another method is to hold the second page in the printer for a predetermined amount of time until the first page has had time to dry completely. Obviously this decreases the printer""s throughput rate.
What is needed in the art is a method of determining when ink on the first page is dry so that the second page may then be ejected from the printer, thereby optimizing printer performance.
The present invention provides a method and sensor apparatus for accurately detecting, under various conditions, whether ink is dry on a print medium.
The present invention comprises, in one form thereof, a dry time sensor apparatus for an ink jet printer. A light source emits light onto a selected area of ink on a print medium such that the light reflects off of the selected area of ink. A reflective device receives the reflected light and reflects the light a second time back onto the selected area of ink such that the light is reflected a third time by the selected area of ink in a predetermined direction. The predetermined direction is substantially nonvarying over a range of angles of orientation of the print medium and a range of distances of the print medium from the light source. A reflected light detecting device receives the light reflected in the predetermined direction.
It is easy to look at the surface of wet ink and determine that it is wet. This judgment is commonly made based on the shininess of the surface. An ink dry time sensor is essentially designed to allow some objective measurements of shininess (gloss) of a given area to be made over time. The general idea is that the drying of the ink/paper can be correlated with these measurements of shininess. An ink dry time sensor attempts to measure the light that is reflected and scattered from the surface of a page onto which ink has been applied in some pattern (the print pattern).
A light source is directed to the surface of the paper and some number of photo sensors are placed so as to measure the amounts of the reflected light in various positions over time. Generally, one photo sensor is positioned such that it receives a peak reading if all of the light is spectrally reflected from the surface of the page. Ideally, the reflective sensor should be placed such that its centerline is coincident with the centerline of the illumination ray after it is reflected from the surface of the paper. The assumption being that the paper (with wet ink on its surface) might reflect the light in the same way as a perfect mirror might.
One potential problem with the sensor is that the measured amount of specularly reflected light can vary dramatically if the sensor moves relative to the paper or if the shape of the paper changes during its operation. The positioning if the sensor relative to the paper is largely a matter of accurate feeding of the paper and mounting of the sensor. The movement and deformation of the paper is a common occurrence resulting from the relaxation of mechanical stresses in the paper as the ink is absorbed (cockle and curl). The variations in the measurements from the reflective sensor due to these paper movements can easily overshadow the signal in which we are interested.
The present invention minimizes the effects of positional tolerances and paper deformation by using a beam splitter and a retro-reflective surface.
The method of the present invention provides a theory for the operation of the ink dry time sensor and processes that can be used to determine the xe2x80x9cdry timexe2x80x9d (and associated information about drying) from the data supplied by the sensor hardware.
An advantage of the present invention is that the sensor can compensate for small deformations and movements of the paper. Thus, the sensor is accurate and easy to use.
Another advantage of the present invention is that ejected sheets of paper are stacked as soon as the ink is dry, thereby maximizing the printer""s rate of throughput and minimizing resulting smear and offset print quality defects.